The present invention relates to a method of recording and/or reproducing a signal, a method of reproducing a signal, and a method of recording/reproducing a signal.
A PCM (pulse-code modulation) acoustic synthesizer circuit is known which reads a digital signal at certain intervals of time from a ROM or the like in which the digital signal corresponding to amplitude values of an acoustic waveform such as audio speech is stored. Then, the circuit converts the digital signal into analog form and reproduces the acoustic waveform into, for example, an audio speech.
Recently, a DPCM (differential pulse-code modulation) signal recording/reproducing method has become available as a method of recording and reproducing an analog signal with a fewer number of bits than the conventional PCM method. In particular, whenever an analog signal is sampled, the difference between the presently sampled amplitude value and the previously sampled amplitude value is taken. The resulting value is stored as a digital signal in a ROM or the like. During reproduction, whenever the digital signal is read from the ROM, this digital signal is accumulated to the digital signals read out thus far. A digital signal corresponding to an amplitude value of a sample which results in the currently read digital signal is calculated. This calculated digital signal is converted into analog form. Thus, an analog waveform is obtained.
It is known that in the PCM and DPCM methods, if a signal exceeding the full-scale voltage is applied during analog-to-digital conversion, an overflow occurs, and that the recorded and reproduced waveform is distorted due to overload noise. The prior art DPCM method cannot follow rapid waveform variations because of limited quantized bit numbers. In order to avoid this situation and to record and reproduce an analog signal having a wide dynamic range, if the full-scale voltage is enlarged and the number of quantized bit numbers is increased, then the merit of the DPCM method, i.e., capable of recording and reproducing an analog signal with limited quantization bit numbers, is defeated.
Furthermore, if the number of quantization bits is increased, the amount of information per digital signal is, of course, increased. Where the waveform variations are mild, the value of the digital signal indicating a difference decreases. As a result, upper bits become vacant, i.e., assume a value of 0. For example, when the number of quantization bits is 8 bits, if the digital signal indicative of the difference value is "00010111", then the upper three bits are vacant. That is, effective use of the storage capacity of the ROM or the like cannot be made.